The present invention relates to electrical impedance, more particularly to methods and apparatuses for measuring the electrical impedance of a thin film of fluid.
Electrical impedance, typically designated “Z” and expressed in ohms, is the amount of resistance of an electrical circuit to current flow upon the provision of a voltage across a pair of terminals of the electrical circuit. Basically, the impedance of the electrical circuit is the ratio of the voltage across the terminals to the current flow between the terminals. Impedance in direct current (DC) circuits and alternating current (AC) circuits are analogous but differ. In DC circuits, impedance corresponds to resistance, wherein voltage “V” equals the product of the current “I” and the resistance “R.” In AC circuits, impedance can be more complicated, as it depends upon the combined effects of all of the circuitry components. For instance, inductors and capacitors accumulate voltages that oppose current flow of current, and these inductive and capacitative reactances are to be considered along with resistance in order to find the impedance.
As a material property, impedance represents the opposition of a material to current flow, and normally encompasses the effects of both conductivity and permittivity on the current flow. Properties of materials are usually expressed in “bulk” form. Testing of bulk material properties may involve, for instance, the placement of a large, homogeneous piece of material in an apparatus. Bulk material properties are useful for most but not all applications. Generally speaking, the term “thin film” is conventionally applied to a material layer that is very thin. Many so-called “thin films” range in thickness between fractions of a nanometer to several micrometers.
Testing of properties of a “thin” material is quite unlike testing of properties of a bulk material. As the thickness of a material approaches zero, the internal electrical interactions which determine its material properties change, and the “thin” material properties (e.g., “thin film” properties or “thin wire” properties) increasingly dominate. Generally speaking, the testing of “thin film” properties or “thin wire” properties for solid materials is easy and straightforward. In contrast, the testing of “thin film” properties for fluid materials is difficult and problematical, because surface tension controls the thickness of a pool of fluid. In the case of a bubble, for instance, it is difficult to test the fluid's properties without “bursting the bubble.”